Various means were used in the prior art to determine when a metal surface should be removed from an etchant solution so that it would be properly prepared for resistance welding and or adhesive bonding. In the past, the time that a material was to be etched was empirically determined by evaluating a test cupon after it had been exposed to the etchant for a specific internal time. The suitability of the particular time chosen for the test specimen to remain in the etchant was generally evaluated by resistance welding and or adhesively bonding representative cupons, and then destructively testing them to select a processing condition that gave the most desirous test characteristics.
Frequently, the prior art test devices and methods were unsatisfactory because relatively large quantities of material could be processed before a deficiency was detected. The lack of a rapid sensing cell to monitor the process often resulted in the production of material which produced weak bonds. The need to inspect and remove such defective material from an assembly line generally resulted in waste of material, time and labor.